While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
Typical phosphorescent dopants are orthometallated Ir or Pt complexes, such as fac-tris(2-phenyl-pyridinato-N^C-)Iridium(III) (Ir(ppy)3) that emits green light (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H.-B. Bürgi, Inorg. Chem., 33, 545 (1994)). Organic electroluminescent devices having high efficiency have been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)).
An amido-bridged bimetallic copper system, {(PNP)Cu1}2, derived from a chelating bis(phosphine)amide ligand ([PNP]-=bis(2-(diisobutylphosphino)phenyl)amide) was reported by Seth B. Harkins and Jonas C. Peters, J. Am. Chem. Soc., 127, 2030-2031 (2005) to emit strongly in solution and in the solid state when irradiated by visible light. Electrochemical analysis of {(PNP)Cu1}2 in CH2Cl2 (Fc+/Fc, 0.3 M [nBu4N][PF6], 250 mV/s, Fc=ferrocene) revealed two reversible waves, one centered at −550 mV and the other at 300 mV. A value of −3.2 V (vs Fc+/Fc) was estimated for the excited-state reduction potential of {(PNP)Cu1}2, suggesting that it is possible that {(PNP)Cu1}2 will prove to be a potent photoreductant/photosensitizer, and given the presence of two reversible redox couples within this bimetallic copper system, there may be an opportunity to drive multielectron reaction processes.
In selecting materials for use in an OLED device, in particular as a phosphorescent emitter, it is not sufficient that the material is emissive when irradiated by visible light. The excited state must be formed by applying an electric potential to an OLED, and further, the excited state must emit light from the OLED device rather than simply reacting chemically with the surrounding materials comprising the device structure.
Iridium or platinum organometallic compounds have been the predominant triplet dopants proposed for use in OLED devices. It would be desirable to have alternatives to iridium or platinum organometallic compounds for use as triplet dopants. New compounds need to be discovered in order to improve the properties of OLED devices such as efficiency and color. In addition, iridium and platinum are particularly expensive metals.